Digital oscilloscopes operate by sampling a time varying analog signal periodically and storing the sample values in correlation with time in a waveform memory. These data may be then read out as locations in the memory are sequentially addressed by a clock signal to provide digital data which can be converted by a digital-to-analog converter to a time varying output signal which can be displayed on the face of a cathode ray tube (CRT). The waveform data may be further processed and placed in a display memory and the data in the display memory read out in the proper fashion to create the display on the CRT. The selection of the portion of the analog input waveform which is sampled and stored is determined by appropriate triggering circuitry so that the operator will have available the desired portion of the waveform which he wishes to review.
The waveform data stored in the waveform memory typically consists of a large number of data points (e.g., 50,000) which corresponds to the magnitude of the analog input signal which has been sampled at a sample rate over a selected period of time. If the full memory of data, constituting the full waveform which has been captured and stored, is read out, the entire waveform will be displayed extending over the width of the CRT screen.
When the entire captured waveform is displayed on the CRT face, the features of the waveform may be drawn so closely together that these features may be difficult to distinguish on visual examination. The density of the waveform as displayed may be such that the operator cannot conveniently use cursor or marker functions which are commonly found on digital oscilloscopes for measuring the magnitude of the waveform at particular points on the waveform. One way in which this problem can be simply resolved is to display only a portion of the full waveform on the CRT screen at any one time, thereby horizontally expanding the time base for the displayed waveform and more fully separating the data points that are displayed. This can easily be accomplished by, for example, reading out the data for the selected section of the waveform stored in memory at a slower rate compared with the movement of the electron beam horizontally across the CRT face to thereby spread further apart the points on the CRT screen which are illuminated. This technique corresponds roughly to speeding up the time base of the sweep in a conventional analog oscilloscope.
U.S. Pat. No. 3,859,556 to Schumann, the disclosure of which is incorporated herein by reference, discloses means for selecting any one of the displayed data points and providing an expanded display in which the selected point is at the center of the display and the number of adjacent data points which are displayed is determined by the selected point and the magnification factor. The selected point can also be varied during expansion.
One problem with displaying a portion of the stored waveform on an expanded time base is that the operator may not readily be able to determine the position within the full stored waveform at which the portion of the waveform being displayed is located. This problem has been addressed by displaying the full waveform data and simultaneously displaying the expanded portion of the waveform on the CRT screen. The position of the expanded waveform within the full waveform can be indicated by a cursor located on the full waveform. This cursor can be moved to change the location of the expanded portion of the waveform. In this approach, the expanded and unexpanded waveforms may be superimposed upon one another, which can interfere with visual examination of the waveforms. Horizontal expansion of the portion of the waveform may be limited to a fixed amount of expansion so that the operator may obtain an expanded waveform which is a greater or lesser portion of the waveform than he wanted. If the expanded part of the waveform is greater than the portion that he wanted, and if the waveform is sufficiently rapidly varying, fine detail features of the waveform may still be difficult to read on the expanded waveform. If the waveform memory is relatively large, it may contain much more data than the operator would ordinarily wish to look at as a full waveform. In such a case, the operator's main interest may be to look at portions of the waveform while nonetheless being able to review the full waveform simultaneously so that he can observe any long term trends in the waveform as well as significant transient changes in the input signal over the period of time during which the signal was sampled and stored.